This application claims the priority of German patent document 198 22 860.0, filed May 22, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention first relates to a method and apparatus of braking a vehicle by means of at least a service brake and a retarder brake, in which the brake pedal velocity is detected, and a control device increases the applied brake pressure if the brake pedal velocity exceeds a definable triggering threshold value.
A method and apparatus of this generic type for actively assisting a driver in emergency situations (known as a "brake assist") is disclosed, for example, in German journal ATZ Automobiltechnische Zeitschrift 99 (1997), 6, Page 330, and on. This brake assist utilizes the recognition that, on average, brake pedal velocities in emergency braking situations are higher by a factor 3 than in all other braking operations. The detection of the brake pedal velocity therefore permits a simple and reliable recognition of such a situation. The control unit, in coordination with an anti-lock brake system, increases the brake pressure as a function of the brake pedal velocity, to achieve optimal utilization of the vehicle braking potential for each driver, even in emergency braking situations.
In addition to the service brake, utility vehicles also have a retarder brake, which can be operated by a separate operating element, and which is constructed, for example, as an eddy current brake or as a hydraulic brake. In most cases, the driver first uses the no-wear retarder brake for braking the vehicle. When an emergency situation occurs, the retarder brake is often operated in parallel to the service brake. It was found that, when the retarder brake and the service brake, which can be operated by the brake pedal, are operated simultaneously, the brake pedal is clearly operated more hesitantly than when only the service brake is operated. As the result of such a faulty operation of the vehicle by the driver, which is inappropriate for the situation, the technically existing deceleration potential of the vehicle is not fully utilized.
It is therefore an object of the invention to provide an improved method and apparatus for braking a vehicle by means of a service brake and a retarder brake of the above-mentioned type, which achieves optimal use of the technically possible deceleration potential of the vehicle when the service brake and the retarder brake are operated simultaneously.
These and other objects and advantages are achieved by the method and apparatus according to the invention, in which the position of an operating element of the retarder brake is detected and the activation threshold value is reduced when the position of the operating element exceeds a definable limit value.
By detecting the position of the operating element of the retarder brake and reducing the activation threshold of the brake pressure when this position exceeds a definable limit value, the brake assist can be activated at a lower pedal velocity even when the retarder brake is operated simultaneously, in a manner which technically is easy to implement. As the result, the faulty operation of the vehicle by the driver which has been observed in many simulator tests (and which consists of the fact that, if the retarder brake is operated, drivers of a utility vehicle or of a tour bus step less hard and less fast onto the brake pedal of the service brake) can be counteracted in a particularly advantageous manner.
In most cases, the retarder brake is operated by means of a lever arranged on the steering wheel. Accordingly, in an advantageous embodiment of the invention, either the operating path of such a retarder lever is detected, or the control signal for the retarder triggering is detected.
The activation threshold and the limit value of the operating path of the retarder lever are advantageously determined, for example, by simulation tests. An advantageous embodiment provides that the activation threshold is reduced by 20% to 40% (preferably by 30%) when the operating path exceeds a limit value of 67% of the maximal operating path.
The invention also provides a device for braking a vehicle by means of a service brake and a retarder brake, having a sensor for detecting the brake pedal velocity and having a control unit which increases the brake pressure if the detected brake pedal velocity exceeds a definable triggering value. In this manner, the invention provides a system which is technically simple. When the service brake and the retarder brake are operated simultaneously, the control unit determines and emits a control signal for brake pressure which results in an optimal deceleration of the vehicle. For this purpose, another sensing device detects the position of an operating element of the retarder brake, and provides a signal to the control device, which can reduce the triggering threshold value if the position of the operating element exceeds a limit value.
It is particularly advantageous that this arrangement requires only one additional sensor for detecting the operation of the operating element of the retarder brake. The signal supplied by this sensor can be processed in the control unit of the brake assist, which is already present, and can therefore virtually be displaced to the "program level".
It is preferably provided that the operating element is a retarder lever, and that the additional sensing device detects the lever path of the retarder lever.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.